1. Field of the Invention
This invention relates to class D power amplifiers and more particularly relates to class D power amplifiers for use in low power applications such as in battery powered systems where thermal dissipation is a factor.
2. Description of the Prior Art
Often, audiophile applications use class A or class AB amplifiers to minimize distortion, notwithstanding the high inefficiency of class A and class AB amplifiers. Thus, such amplifiers consume a relatively large amount of power for the amount of power provided to the output. Typically, such amplifiers will dissipate heat energy at least equal to the amount of energy being provided to the load. Thus, cooling of such class A or class AB amplifiers is often an important consideration and requires large (and generally heavy) heat sinks and/or cooling fans. Nonetheless where AC power is available and where weight and size are not crucial factors, high fidelity audio amplifiers use such class A and class AB amplifiers for audio amplification.
Due to their high fidelity, such class A and class AB amplifiers are also used in conventional portable applications such as portable compact disc (CD) players, portable tape players and notebook and subnotebook computers. While such class A and class AB amplifiers will provide output signals with relatively high fidelity, such class A and class AB amplifiers can provide only limited output power. Otherwise, due to their inefficiency, they draw too much power for long term battery operation. Still further, such class A and class AB amplifiers dissipate too much power as heat at high power applications. Heat dissipation in many portable applications such as portable CD players and portable computers creates severe problems in such small units. Therefore, in such portable applications, the power amplifiers are typically limited to less than one-half a watt of output power. As a result, any sound produced by speakers directly driven by such amplifiers will have a relatively low loudness.
However, there has been a recent trend towards using portable computers for multimedia presentations. Since the audio output of the computer is limited due to the restrictions on the power output of the amplifiers, such computers can generally only be used for presentations to a few people. Alternatively, separate powered speakers can be used, but such speakers are bulky and heavy. Therefore, they are often undesirable, not only for laptop and other portable computers, but for desktop and other nonportable computers.
While it might appear that a class D amplifier might be used in lieu of a class A or a class AB amplifier, such class D amplifiers suffer from a number of drawbacks that have limited their utility in audio applications. FIG. 1 shows a typical class D amplifier 10. The input signal 12, which may be an audio signal, is provided to the non-inverting input of a comparator 14. The inverting input of the comparator 14 is coupled to a triangle wave generator 13 that generates a triangle wave symmetric to ground. The output of the class D amplifier is used to drive a switching bridge circuit such as the bridge circuit 16 shown in FIG. 2 where a first transistor pair Q1 and Q4 are controlled by a driver circuit 15 to conduct simultaneously and a second, complementary transistor pair Q2 and Q3 are controlled to conduct simultaneously. When transistor pair Q1 and Q4 conduct, current from the power supply voltage flows through the loudspeaker 18 in a first direction and when transistor pair Q2 and Q3 conduct, the current through the loudspeaker reverses. Further the transistor pairs are complementary in that when transistor pair Q1 and Q4 conduct, transistor pair Q2 and Q3 do not conduct and vice versa. Thus, the two transistor pairs comprise a full bridge switching circuit. A filter circuit 19 comprised of inductors and capacitors (not shown) serves to transform the current into a substantially sinusoidal signal that varies with the input audio signal, and thereby substantially reproduces the input signal, theoretically.
However, in practice, there are a number of problems associated with such prior art class D amplifiers that prevent the class D amplifier from accurately reproducing the input waveform except for low fidelity applications. In particular, as a practical matter, it is difficult to generate a symmetrical and linear triangle wave. Any lack of symmetry or non-linearity in the triangle wave's waveshape introduces distortion into the output signal. Further, the comparators 14 are subject to switching transients at about the crossover points for the amplitude of the input signal versus the triangle wave. For example, noise and other fluctuations on the signal may cause the output of the comparator to switch back to the prior state temporarily, injecting noise into the output signal.
Still further, the class D amplifier is also subject to power supply perturbations. As the power supply voltage varies, the gain of the amplifier varies. This causes potential frequency instabilities in the performance of the class D amplifier such as in the filters necessary to reproduce audio.
As a result, class D amplifiers are typically not used in applicatiorns where good fidelity is demanded such as in portable compact disc players or on notebook computers used for multimedia presentations where total harmonic distortion should be preferably less than 1%. Rather, class D amplifiers are typically used in other applications such as hearing aids where fidelity is not a concern or for driving woofers or subwoofers where the low frequencies mean that typical class D amplifiers will have better performance.
A second prior art audio class D amplifier is shown in FIGS. 2A, 2B, and 2C which is described partially in each of Harris Corporation Application Note No. AN9525.2 dated March 1996 for the Harris Class D Audio II Evaluation Board (HIP4080A EVAL2) and the Application Note AN9404.1 dated March 1995. FIG. 2A shows an audio input 51 that is coupled through an analog summing network 52 to provide an input signal to the pulse width modulation (PWM) comparator 72 (FIG. 2C). The summing network 52 level shifts the input signal to be centered about 6 volts, and sums the audio input signal with the feedback current and a current limit set to limit the output drive current through the bridge 62 in (FIG. 2B). In addition, a symmetrical triangle wave signal is applied to the +input of the PWM comparator 72 to provide pulse width modulation of a symmetric triangle wave. Referring to FIG. 2C, the PWM comparator 72 is applied to delay circuitry 63a and 63b that controls the switching of the bridge transistors so that when transistors Q2 and Q5 are conducting, transistors Q3 and Q4 are not conducting and vice versa. The level shift circuitry 64 is used for shifting the signaling voltages for controlling the high MOSFET's Q2 and Q4 and further "latching" circuitry is used to keep the high transistors Q2 and Q4 conducting even though the output pulse from the level translation circuitry is short for purposes of power reduction.
However, circuits such as those described above, have a number of disadvantages. First, in portable applications such as laptop computers, AC and DC power supply variations cause significant problems in the stability of class D amplifiers. For example, in typical class D amplifiers, the class D amplifier is subject to gain variations as the power supply varies. As the power applied to the sources of the high transistors increases such as transistors Q1 and Q3 above, the gain of the amplifier increases. This can also cause frequency instability in the output filter.
Therefore, the usage of class D audio amplifiers in battery applications has generally been limited to cases where the battery output is stable such as in automobile applications. Further, such class D amplifiers are typically used for high power subwoofers for automobile stereos rather than for broad band audio applications. Gain variations due to power supply variations is not generally a significant factor because the large batteries and the alternators in automobiles generally provide substantially constant power. Further, since the frequency of application is relatively low, under 200 Hz for a typical subwoofer, frequency instability is not a significant problem. Given that low frequency sound is generally omnidirectional, only one channel is needed for the amplifier. Also, in typical automobile applications, power dissipation and form factor is generally not a problem.
As yet an additional problem, in typical portable audio applications such as in notebook and subnotebook computers and in portable compact disc players and tape players, class D amplifiers have not been generally practical. With stereo sound, two channels of audio are needed and there is a need to have the two channels synchronized. With circuits such as the Harris circuits discussed above, the controllers are single channel controllers and synchronization is not believed to be facilitated due to noise and other factors. Also, heat dissipation and form factor are problems as well. Further, given that digital audio provides signals at frequencies of up to 22 KHz, gain stability and power supply effects on stability are believed to be far more problematical.
Still further, the AC and DC variation of the power supply in portable computing applications is also potentially quite large. Since many portable computers and portable music devices are designed to support multiple chemistries for the batteries such as NiCd, lithium hydride and nickel hydride, the available power supply can vary between for example 8 volts DC to 25 volts DC depending upon the battery being used. Further, as the battery is discharged, the DC voltage level varies substantially, particularly as the battery nears exhaustion. In addition, AC variations in the battery can be substantial due to the switching power supplies that are used for charging the batteries. Thus, AC fluctuations of several volts can be seen in the power supply voltage in common battery supported applications.
Therefore, it is a first object of the invention to provide a small form factor broad frequency range audio power amplifier. It is a second object of this invention to achieve the audio amplifier with reduced heat dissipation and reduced power draw on the power source. It is yet a third object of the invention to provide DC power supply rejection in such amplifier to minimize gain fluctuations with power supply fluctuations. It is yet a fourth object of the invention to provide such an amplifier with reduced distortion of the signal.